Telephone system



Nov. 10, 942- E. P. FAIRBAIRN I 2,301,530

TELEPHONE SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 1 [0 I F157. 1 D

TELEPHONE IA/l 111+ A [III INVENTOR ERC PLAYFMR FMRBAIRN ATTORNEY Nov.10, 1942. FAlRBAlRN 2,301,530

TELEPHONE SYSTEM Filed Oct. 7, 1940 2 Sheets-Sheet 2 INVENTOR ERCPLAYFAIR FAIRBMRN ATTORNEY Patented Nov. 10, 1942 TELEPHONE SYSTEM EricPlayfair Fairbairn, Coventry, England, as- Signor to The GeneralElectric Company Limited, London, England, a British company ApplicationOctober 7, 1940, Serial No. 360,099 In Great Britain October 31, 1939 3Ciaims.

The present invention relates to loud-speaking telephone systems, andhas for its object the provision of means for enabling both wayloud-speaking facilities to be provided by the elimination of certainelectrical difiiculties previously encountered.

The invention is more particularly applicable to loud-speaking systemsemployed in connection with private automatic exchanges. in theseexchanges the line resistances and impedances are all very similar, asalso are the impedance characteristics of the subscribers instruments.

In order to give loud-speaking facilities on any instrument employingonly a two-Wire interconnecting channel, it is necessary to provide amicrophone and amplifier the output of which is applied to the linewires in such a way that substantially none of this output is fed backinto the local; loud-speaker circuit. This is normally effected bypassing the output of the microphone amplifier through a balancedtransformer in which equal proportions of the output are fed to the lineand. to a balance network, this latter being arranged to simulate asclosely as possible the impedance characteristics of the line, exchangetransmission bridge, and subscribers termination as seen from theloud-speaking termination. Connections to the local loud speaker arethen derived from equipotential points in the transformer windings.

Such a system operates correctly provided that the line and instrumentimpedance found on any call made from the lou -speaking instrument issubstantially constant. If, however, an attempt is made to interconnecttwo such loud-speaking terminations it has hitherto been found that thevariation in impedance between such a loud speaking termination and anordinary subscribers instrument has been too great to permit more than arelatively small amount of overall amplification of speech, since ifmore than this critical value of gain were attempted, an oscillation dueto out-of-balance conditions would be obtained.

According to the invention, such out-of-balance conditions are preventedin a system of the type described, by arranging a compensating networkin the line circuit of each loud-speaking equipment, this networkcausing the characteristic impedance of the loud-speaking equipment toresemble closely that of the normal subscribers instrument termination.

The loud-speaking instrument is then provided with a line balance whichcan be made to approximate very closely to the impedance characteristicfound on any call, whether to a normal subscriber or to anotherloud-speaking installation, this greater accuracy of balance permittinga higher electrical gain and much greater stability than was previouslythe case.

In loud-speaking instruments of the type described, it is desirable toarrange that speech may be transmitted and received within a fraction ofa second of the subscriber pressing the reply or calling button orswitch. Even with directly heated valves, such a condition is not easilymet where alternating current derived from a transformer or directcurrent through a choke or barretter is employed for filament heating.it is therefore arranged according to a feature of the invention thatthe filaments of the valves of a -loud-speaking installation are heateddirectly upon switching-on of the installation by a current considerablyhigher than that of their normal running value, the magnitude of thiscurrent being reduced to normal directly an adequate filamenttemperature is reached.

When alternating current is employed, a suitable arrangement consists ofa relay connected across the valve high-tension supply. The filaments ofall valves are initially heated by alternating current considerably inexcess of their normal; value, and as soon as high tension is available,operation of the relay either reduces the alternating current to anormal value or changes over the filament supply to a direct currentsource, supplying current of the correct figure.

If D. C. power mains are employed, the relay is connected across thevalve filaments, which in turn are connected in series with a barretter.ihis latter is initially cy-passed by an ordinary metal filament lamp.Both this lamp and the valve filaments, when cold, are of relatively lowresistance, so that immediately on switching on the valves, the valvefilaments substantially short-circuit the relay. The valve filaments areheated rapidly in series with the metal filament lamp, and when theyreach their working temperature sufiicient current is diverted by theirincreased resistance through the relay to operate it. This actiondisconnects the metal filament lamp and allows normal current throughthe barretter to keep the filaments at their correct temperature.

The subscribers telephone instrument is provided with a signalling lampand as the instrument is placed close to the subscriber, and is handledby him, it is arranged according to a further feature of my invention,that no direct connec tion from the said instrument is made at any timeto the supply mains, if used. For this purpose, transformers areinserted at suitable points to isolate the microphone, loud-speaker andhand micro-telephone, and it is arranged that the signalling lamp is liteither from low voltage A. C. when the supply is of this nature, or fromalternating current derived from a valve oscillator when D. C. mainsonly are available. In each case the transformer winding supplying thelamp is suitably insulated and/or earthed.

In order that the nature of the invention may be more fully understood,reference should be made to the accompanying drawings which showparticular embodiments of my invention.

Fig. 1 shows a telephone line terminating in a substation including loudspeaking telephone equipment.

Fig. 2 shows one arrangement for supplying power to the valves of theamplifier.

Fig, 3 shows another arrangement for controlling the power supply to avalve.

Fig. 4 shows a method of supplying a low voltage to the filament of alow voltage lamp when the main supply is of high potential directcurrent.

In Figure 1, line wires LI and L2 are connected to a compensatingnetwork comprising an aircored inductive impedance ACI, an attenuatingnetwork consisting of four non-inductive resistances RI, R2, R3 and R4,a condenser Cl which isolates the line current, and a hybrid coil I-IChaving line windings LWI and LWZ, line balance windings LBI and LE2 andmicrophone input winding IW, which is connected to a microphone M bymeans of conductors IA and IB. A holding coil H maintains a loop acrossthe line wires for supervisory purposes. Connected to the line balancecoils is a line balance network having linear impedance elementsarranged to simulate the impedance/frequency characteristic of thecircuit terminated across terminals LI and L2. The line balance networkcomprises an air-cored inductance AC2, non-inductive resistances R5, R6and R7, condenser C2 and an iron-cored inductance LA. Conductors IC andID are connected to the input terminals of a loud-speaker amplifier(shown diagrammatically), a non-inductive resistance R9 forming a lowresistance terminating impedance in case the amplifier input impedanceis high, and the output of the amplifier is shown connected to a loudspeaker LS.

In Figure 2, an iron-cored transformer T having a primary winding Pconnected by conductors MA and MB to a supply of alternating current hasthree secondary windings S2, S3 and S4. A supply of direct current whichis provided by means of secondary winding S3 and a rectifier valve VA isconnected to the high-tension supply terminals of an amplifier such asused in the telephone circuit of Fig. 1 via conductors HT+ :and HT. Thefilament of the rectifying valve VA is heated by means of an alternatingcurrent provided by the secondary winding S2. A similar supply ofalternating current is provided by means of the secondary winding S4 tofilaments of the amplifier valves of such amplifier over conductors FAand FE. The potential across the filaments of the rectifier andamplifier valves when supplied by the whole of the windings: S2 and S4is in excess of the required normal working potential in order that thefilaments shall be heated quickly. When the rectifier valve isfunctioning correctly, the potential across the, conductors HT+ and HT-to the amplifier reaches a value which causes relay A to operatewhereupon contacts Al reduce the number of turns in the portion ofwinding S2 and contacts A2 reduce the number of working turns in windingS4 so that the potentials to the valve filaments are reduced to thevalues required for normal working.

Referring now to Figure 3, another method for reducing filament currentto an amplifier is shown wherein a source of high-potential directcurrent connected to the conductors DC+ and DC' heats the filament of athermionic valve VB of the amplifier in series with a barretter BR and ametal filament lamp PL. The resistance of the valv filament when cold islow enough to substantially short-circuit the relay B. When the powersupply is switched on, the valve filament heats to its normal workingtemperature very rapidly whereupon the resistance of the filamentincreases and relay B operates. Contacts Bl disconnect the lamp so thatthe amount of current passing through the valve filament is reduced tothe amount required for normal working and is limited by the barretter.

In Figure 4, a local oscillating valve circuit consisting of athermionic valve VO, condensers PC and G0, a resistance R3 and aniron-cored output transformer T having windings T I, T2 and T3, receivesa direct-current high tension voltage supply over the two conductors andA low voltage alternating current is induced into winding T3 and fed toan indicating lamp over conductors FA and FE.

I claim:

1. In a telephone system, a telephone line having loud speakingequipment including a loud speaker and a microphone associatedtherewith, a hybrid coil connecting said equipment to said line, acompensating network connected between the coil and the line circuitoperative to cause the characteristic impedance of the line and the loudspeaking equipment to resemble closely the impedance of a line having aregular substation telephone, and a line balancing network associatedwith said loudspeaking equipment through said coil to balance the lineimpedance.

2. In a telephone system, a line terminating in a substation having loudspeaking equipment in cluding an amplifier, a hybrid coil connectingsaid equipment to the line and having a balanced network which balancesthe impedance of the line, and a compensating network between the coiland the line for bringing the impedance of the line having the loudspeaking equipment approximately equal to the impedance of a line havinga standard transmitter and receiver thereon.

3. In a telephone system, a line having loudspeaking equipment includinga loud speaker, a microphone and an amplifier connected thereto, acompensating network to cause the line to resemble a regular substationtelephone line and a network to balance the line impedance, wherein theamplifier has filament type valves therein, and means for supplying aninitial high current to said valves and means for cutting down thecurrent thereto after an interval.

ERIC PLAYFAIR FAIRBAIRN.

